1. Field of the Invention
The field of the present invention is that of bypass turbojets, and more particularly that of the afterburner devices of such jets.
2. Description of the Related Art
In a bypass turbojet with afterburner, with an afterbody of the type illustrated in FIG. 1, the airflow is first of all aspirated by a low-pressure compressor. A first portion of this airflow on leaving the low-pressure compressor feeds a high-pressure compressor, while a second portion passes into a first passageway 1 defined between an outer annular housing 2 and a first inner annular housing 3. The airflow compressed by the high-pressure compressor feeds a combustion chamber which itself feeds combustion gas to a high-pressure turbine followed by a low-pressure turbine and the outlet of which passes through a second passageway 4 defined between the first inner annular housing 3 (or confluence sheet) and a second inner annular housing 5 (or exhaust cone).
The combustion gases that feed the second passageway 4 have a high temperature and constitute what is called is a main flow (or hot flow). The air that feeds the first passageway 1 has a temperature that is substantially lower than that of the main flow and constitutes what is called a bypass flow (or cold flow).
Downstream of the turbine outlet, it is possible to produce an increase in thrust by injecting an additional quantity of fuel into the main and bypass flows, with its combustion in an afterburner channel. This system comprises mainly a set of arms 7, called flame-holder arms, and a spray ring 6. The spray ring 6 is supported by the arms 7 and placed in the bypass flow, in the vicinity of the confluence sheet 3.
A portion of the injection takes place with the aid of the spray ring 6 which makes it possible to inject a portion of the fuel evenly and to stabilize the flame.
Moreover, each arm contains a fuel injector which injects, into the main flow, the other portion of the afterburner fuel.
The arm is also fitted with a ventilation tube the role of which is to cool on the one hand the metal walls of the arm that are swept by the main flow, and, on the other hand, the injector, air originating directly from the cold flow.
The injector-tube assembly is itself protected from the radiation of the flame by another metal part, the heat shield.
In the prior art, the arms 7 were initially made of metal, and cast. This configuration had the drawback of having to ensure that they were cooled. In more recent implementations, they have been replaced by parts made in ceramic matrix composite (CMC) material which have the advantage of being lighter and more resistant to high temperatures. It is then possible to dispense with the ventilation systems to which the metal systems are condemned.
On the other hand, these materials have thermal expansion characteristics that are very different from those of the materials constituting the engine housings to which they are attached. It is therefore necessary to take account of the effects of these differences in the way the arms 7 are attached to the structure of the engine, notably to the outer housing 2.
Several devices have been proposed for attaching the arms 7, whether the latter are made of metal or of composite material. They are described in documents FR2699226, FR2699227 or else in documents U.S. Pat. No. 5,103,638, GB 2 295 214, U.S. Pat. No. 5,022,805 or U.S. Pat. No. 5,090,198.
Document FR2865502 by the applicant describes, for its part, a one-piece flame-holder arm 7 made of composite material (ceramic matrix composite, or CMC) in order to withstand the high temperatures, which is attached directly to the outer housing 2. This arm has the shape of two walls joined together in the bottom portion by a throat, with, in the top portion a recess that is open to the cold flow and two curved flanges. The attachment is achieved via these two flanges which are bolted to the housing, which, during manufacture of the arm, requires the folding of the fibres of the composite material in order to give them the appropriate shape. As indicated above, these flanges made of composite material do not have the same coefficient of expansion as the metal of the housing. Consequently, on the one hand, the great difference in expansion between the CMC arm and the metal housing may cause a loss of tightness between the 2 parts when hot, and, on the other hand, attaching the CMC arms to the housing generates stresses in the flanges of the arms. The consequences of these stresses are further amplified by the fragility of the flanges in this zone due to the folding of the CMC fibres. These stresses are quite clearly prejudicial to their service life.
Document EP1803999 by the applicant proposed a solution attempting to solve this problem of stresses by interposing a metal arm support between the arm and the housing, that is to say by attaching the arm to the support and by attaching the support to the outer housing 2. However, the lateral lugs of the support, being brazed to the top plate of the support at the interface with the housing, are subjected to considerable thermal stresses and to considerable aerodynamic forces. The service life of the arms in this configuration therefore remains insufficient because of the great strain on the support at the brazed joints.